This invention relates to an alloy useful for occluding hydrogen in the form of a hydride for the convenience of transportation and storage of hydrogen.
Hydrogen which can be derived from water as the raw material is available in almost infinite quantitites, is clean, is capable of easy transportation and storage and does not through its use disturb the cycles of nature. Because of these advantages, it has come to arrest keen attention as a highly potential fuel of the coming era to take the place of fossil fuels. The time is likely to come when hydrogen is used in huge quantities and in diversified ways. As a consequence, it will become necessary for the supply of hydrogen to satisfy all types of demand without reference to the quantities required or geographical locations. In this connection, the techniques available for the transportation and storage of hydrogen will gain immeasurably in significance.
Among the methods now available for the transportation and storage of hydrogen, there are included those whereby hydrogen is transported and stored in the form of gaseous hydrogen or liquefied hydrogen and those whereby hydrogen is transported and stored in the form of a metal hydride.
Techniques for transporting and storing hydrogen in liquid and gaseous forms have already reached an acceptable, if not ideal, state of development. However, these techniques necessitate use of highly pressureproof facilities and, further, inevitably dictate exercise of great precaution against the hazards arising from the use of elemental hydrogen.
As a technique far advanced over those described above for handling gaseous and liquid hydrogen, the methods which handle hydrogen in the solidified form of a metal hydride have come to attract increasing attention.
The qualities required of a hydrogen occluding metal hydride to be used for transporting and storing hydrogen (such metal hydrides will hereinafter be referred to simply as hydrogen storing materials) can be listed as follows: (1) it must be an occluding metal or alloy which is cheap and abundantly available; (2) it must have a large storage capacity for hydrogen; (3) it must be stable at and near room temperature and have a proper dissociation equilibrium pressure; (4) its hydrogen occluding and liberation reactions must be reversible and progress rapidly and; (5) it must be light of weight.
The materials heretofore recognized as useful for storage of hydrogen by occlusion include a magnesium-nickel alloy [J. J. Reilly, R. H. Wiswall, Jr.: Inorganic Chemistry, 7, 2254 (1968)], a lanthanum-nickel alloy [J. H. N. van Vucht, F. A. Kuijpers, H. C. A. M. Brunning: Philips Research Reports, 25, 133 (1970)] and a titanium-iron alloy [J. J. Reilly, R. H. Wiswall, Jr.: Inorganic Chemistry, 13, 218 (1974)] and the like. Of these alloys, the magnesium-nickel alloy enjoys a relatively large capacity for occlusion of hydrogen per unit weight of the alloy and yet has a disadvantage that the temperature required for the occlusion and liberation of hydrogen is high. In contrast, the lanthanum-nickel alloy and the titanium-iron alloys have low temperatures for the occlusion and liberation of hydrogen. They nevertheless are not completely free from problems which render their actual use disadvantageous such as that the metals contained therein, i.e. titanium, iron, lanthanum and nickel should possess very high purity and the hydrogen itself to be occluded thereby should possess high purity and that the component "lanthanum" is expensive and requires much time for activation. A metal or an alloy which satisfies all the requirements expected to be met by an ideal material capable of providing advantageous occlusion of hydrogen remains yet to be discovered.
The inventors formerly invented alloys of Mischmetal (Mm) with nickel or cobalt as material capable of occlusively storing hydrogen in the form of a hydride and a method for the use of said alloys and applied for Japanese patent on the invention (Japanese Patent Application No. 52927/1975). These alloys enjoy an outstanding characteristic that they are inexpensive, their temperatures for occlusion and liberation of hydrogen are low and the density of hydrogen is practically the same as that of liquefied hydrogen. Comparison of Mischmetal-nickel alloy (MmNi.sub.5) and Mischmetal-cobalt alloy (MmCo.sub.5) in terms of the maximum capacity for hydrogen occlusion (1.5% by weight for MmNi.sub.5 and 0.7% by weight for MmCo.sub.5) and the dissociation equilibrium pressure (14 atmospheres for MmNi.sub.5 and 0.8 atmosphere for MmCo.sub.5) reveals that MmNi.sub.5 hydride contains a large quantity of occluded hydrogen and exhibits a high dissociation equilibrium pressure at room temperature while MmCo.sub.5 hydride contains a small quantity of occluded hydrogen and exhibits a low dissociation equilibrium pressure. Thus, they have mutually contradictory properties. These peculiar characteristics may possibly pose a serious problem, depending on the conditions under which they are actually put to use.
A primary object of the present invention is to provide Mischmetal-containing alloys which have a large capacity for occlusion of hydrogen, permit hydrogen to be occluded therein at a high density, produce hydrides possessed of a proper dissociation equilibrium pressure, possess an ability to liberate hydrogen under mild conditions and prove to be highly advantageous for the practical purpose of stationary storage and transportation of hydrogen.